Why did Lord Krishna run away unable to fight Jarasandhu?

Why did Lord Krishna run away unable to fight Jarasandhu? No, Lord Krishna was not unable to fight Jarasandhu, rather he chose not to kill Jarasandhu. Krishna and Balarama defeated Jarasandhu 18 times. The reason for not killing Jarasandhu was linked to the vow made by Bhima, one of the Pandava brothers. Some more things about Jarasandhu: Jarasandhu was the king of the kingdom of Magadha. He wanted to take revenge from Shri Krishna for the death of his son-in-law Kansa. Jarasandhu was adept at wrestling. Jarasandhu had a boon that whenever his body is cut into two pieces, both the pieces will immediately join each other. Jarasandhu attacked Shri Krishna and Balarama 17 times to kill them. Each time Shri Krishna would destroy his entire army and leave only Jarasandhu alive.